Can someone prove this for me please

Graph g(x)=2cosx . use 3.14 for π . use the sine tool to graph the function. graph the function by plotting two points. the first point must be on the midline and closest to the origin. the second point must be a maximum or minimum value on the graph closest to the first point.

Y= a(x-3)squared-1 what is the value of a

Iknow the answer but the calculator doesn't seem to give the right one